The present invention relates to an electronic camera (i.e., a digital camera) that is provided with an optical element such as a filter between a taking optical system and an image pickup device.
Electronic cameras such as still-video cameras that electronically retain data of freeze-frame pictures or video cameras that electronically retain data of moving video pictures spring into wide use. In the electronic camera, a subject image is formed through the taking optical system on the image pickup device. The image pickup device converts the optical image into electric signals by pixel, and then, the analog electric signals are converted to digital form to be retained in a recording medium.
In general, the electronic camera is provided with a filter such as a lowpass filter, an infrared absorbing filter or the like between the taking optical system and the image pickup device.
The lowpass filter is used to prevent moirxc3xa9 fringes. Since the image pickup device has the pixels that are systematically arranged on a light-receptive surface, when the spatial frequency of the subject image is closely approximate to the sampling spatial frequency that is determined by the pitch of the pixels, the moirxc3xa9 fringes appear in the image data. The lowpass filter reduces the intensity of the spatial frequency component that is close to the sampling spatial frequency. The lowpass filter is a single optical element that consists of a plurality of plates made of birefringent material such as crystal or lithium niobate. In general, the lowpass filter converts the subject image formed through the taking optical system into fourfold images that are deviated to one another by half pitch of the pixel pitch in the four perpendicular directions on the light-receptive surface in order to level out the high frequency component.
Further, the infrared absorbing filter removes the infrared light from the light from the subject. Since the image pickup device senses the infrared light as well as visible light, if the infrared absorbing filter is not provided, the color balance of the reproduced image based on the image data captured by the image pickup device should be different from that of the visual image for a naked eye. The infrared absorbing filter is used to match the color balances between the reproduced image and the visual image.
The various filters are likely to adsorb dust due to static electricity and the dust adsorbed on the filter reduces the quality of the image captured by the image pickup device.
Particularly, when the lowpass filter is made of ferroelecric crystal such as the lithium niobate having pyroelectricity, the filter is likely to adsorb the dust in a camera body because a slight temperature change generates polarization charge on the filter surface.
While the dust adsorbed on the filter can be cleaned by blowing air with a blower or a spray or by wiping with a blush or a cloth, it is necessary to remove the elements such as an interchangeable lens from the camera in order to clean the filter, which takes a lot of time.
Further, even if the filter is cleaned, dust is incorporated in the camera during assembly of the camera and is adsorbed on the filter due to the static electricity. Therefore, the cleaning is ineffectual.
It is therefore an object of the present invention to provide an electronic camera, which is capable of keeping the quality of image captured by the image pickup device even if the filter adsorbs dust.
For the above object, according to the present invention, there is provided an improved electronic camera, which includes: a taking optical system that forms a subject image; an image pickup device that converts the optical image formed thereon into electric signals; an optical element that is located between the taking optical system and the image pickup device, the optical element allowing to pass the light from the subject; and a driving mechanism that moves the optical element in a direction that is substantially parallel to a light-receptive surface of the image pickup device.
With this construction, the light from the subject passes through the taking optical system, and then the light is received by the image pickup device through the optical element that is moved by the driving mechanism. Since the movement of the optical element moves the shadow of the dust formed on the image pickup device, an influence of the shadow is diffused among a larger number of pixels, which reduces the influence upon the specific pixels on which the shadow is formed when the optical element is static. Therefore, the shadow of the dust becomes unobtrusive and there is no appreciable deterioration in image quality.
The driving mechanism may move the optical element when the image pickup device captures an image of a subject. The driving mechanism may consist of a motor and a cam mechanism that converts the rotational movement of the motor into reciprocating rectilinear movement to move the optical element. The driving mechanism may adopt any other mechanisms. For instance, the driving mechanism may rotate the optical element about the optical axis of the taking optical system, or the driving mechanism may move the optical element such that the center of the optical element rotates about the optical axis without changing its orientation. Further, since the driving mechanism is good enough to move the optical element in the direction perpendicular to the optical axis for some distance, the plane in which the optical element moves may be slightly inclined with the light-receptive surface.
The optical element that is moved by the driving mechanism may be a lowpass filter, an infrared absorbing filter or a combination thereof. Since the lowpass filter is designed such that a deviating direction of an extraordinary ray is determined with respect to an ordinary ray assuming that a incident ray is perpendicular to the incident surface, the driving mechanism is required to move the lowpass filter while keeping it perpendicular to the optical axis of the taking optical system.